In a thread grinder configured to grind a thread groove on a cylindrical inner surface of a workpiece such as a thread groove of a ball nut, a grinding wheel is attached to a tip of a grinding wheel shaft, the workpiece is attached to a main shaft facing the grinding wheel shaft, and a rotation axis of the grinding wheel shaft is inclined with respect to an axis of the main shaft in accordance with a lead angle of the thread groove. In the thread grinder, the centering of the grinding wheel needs to be performed with respect to the workpiece before the grinding.
For example, Patent Document 1 discloses a conventional centering method in which a spherical measurement member is attached to a grinding wheel attaching position on the axis of the grinding wheel shaft inclined with respect to the rotation axis of the workpiece and a measuring instrument is held at the main shaft which rotates the workpiece. The measuring instrument is rotated around the measurement member to measure a position of the measurement member by the measuring instrument, the grinding wheel shaft is moved based on the measurement result to position a spherical center of the measurement member on the rotation axis of the workpiece, and then the grinding wheel is replaced with the measurement member and attached to the grinding wheel shaft.